Each time a motion picture camera is started, film is wasted during the period of a few seconds during which the film (and the reels) is accelerated to full speed. For a 70 mm film exposed at 24 fps (frames per second) full speed is about two feet per second, while at 60 fps full speed is about five feet per second. The cost of 70 mm film is about one dollar per foot, so about $10 worth of film may be wasted during a period of four seconds required to accelerate it to 60 fps. The cost can be even higher when a camera is to take pictures for "slow motion," which can require accelerating to an even greater speed. One type of prior art motion picture camera uses a "camera" motor for moving film through the film gate, and a separate "reel" motor for rotating the takeup reel, there often being only portable battery power available for powering the motors. When the takeup reel is empty, the reel motor is capable of rapidly accelerating the takeup reel, while when the takeup reel is almost full, its motor is capable of only slowly accelerating it. However, the takeup reel motor may not be accelerated at the maximum possible rate because of the need to coordinate the camera and reel motors to maintain film tension within a moderate range.
In the past, acceleration of film through the camera was controlled by the camera motor, which was accelerated from zero to full speed at a predetermined moderate acceleration rate, which was slow enough so that the reel motor could keep up with the acceleration rate even under the worst conditions (e.g., the takeup reel was almost full). While the camera motor accelerated at a fixed predetermined rate, the takeup reel motor was slaved to the camera motor. The takeup reel motor was energized at a sufficient acceleration to maintain a moderate film tension along the path between the film gate and the takeup reel. A dancer arm was used to sense film tension. Acceleration was always slow and considerable film was wasted at every startup. A camera which accelerated to full speed about as rapidly as possible under whatever conditions were present (e.g., empty or full takeup reel) would be of considerable value in reducing wastage of film.